1. Field of the Invention
The present invention relates to a wireless communication system and a method of calling a terminal, and in particular, to a wireless communication system and a method of calling a terminal that use acoustic communication by means of an acoustic wave.
2. Description of the Related Art
FIG. 18 is a diagram showing a configuration of an example of a wireless communication system related to the present invention. Referring to the figure, the example of the wireless communication system related to the present invention includes two wireless communication terminals (hereinafter represented as “terminals”) 101 and 102, a wireless communication base station (hereinafter represented as “base station”) 111, and a backbone network 121 connected to the base station 111. These are provided under the sea. Acoustic communication by means of an acoustic wave is used for communication between the terminals 101 and 102 and the base station 111. Although the system includes a plurality of the base stations, illustration of the base stations except the first one is omitted for the sake of convenience. The communication is performed between these terminals and the base stations.
On the other hand, an example of a method of calling a terminal in a wireless communication system including a plurality of terminals and a plurality of base stations is disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2005-354634). This is an art according to which the base station where a call request has occurred uses the CSMA (Carrier Sense Multiple Access) and transmits a call signal from the own station after confirming that any other station does not transmit the call signal, and the terminal on a reception side performs reception only on a prescribed time, that is intermittent reception.
It is an object of the art described in the Patent Document 1 to reduce power consumption on the reception side by periodically receiving a part of times in a basic frame which is formed with a time length of a call signal, instead of receiving all of the basic frame on the reception side.
Furthermore, a method of dividing a call signal into subframes of short time periods and transmitting the subframes is disclosed in Non-Patent Document 1 (A RECEIVER ORIENTED MAC PROTOCOL FOR WIRELESS SENSOR NETWORKS, CAMPELLI, LUCA; CAPONE, ANTONIO; CESANA, MATTEO; EKICI, EYLEM; MOBILE ADHOC AND SENSOR SYSTEMS, 2007. MASS 2007. IEEE INTERNATIONAL CONFERENCE ON 8-11 Oct. 2007, p(s). 1-10). Moreover, an art related to a simple random access system (AROHA system) is disclosed in Non-Patent Document 2 (UNDERWATER ACOUSTIC NETWORKS, ETHEMM. SOZER; MILICA STOJANOVIC, JOHN PROAKIS; IEEE JOURNAL OF OCEANIC ENGINEERING, VOLUME 25, NO. 1, January 2000, p(s). 72-83). Furthermore, among wireless communication networks using an underwater sound, there is an underwater acoustic communication system where every terminal is separately and individually disposed as shown in Non-Patent Document 2.
Moreover, a method of intermittent reception reducing power consumption of a mobile station and downsizing the mobile station is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 05-075523). This is an art that configures a superframe bundling a plurality of frames corresponding to respective time slots of a plurality of groups of mobile stations, varies the number of time slots to be assigned in the superframe on a group basis, and varies intermittent reception rates from one group to another.
Furthermore, another method of intermittent reception reducing power consumption of a mobile station and downsizing the mobile station is disclosed in Patent Document 3 (Japanese Patent No. 3535761). This varies time intervals of intermittent reception in a fashion analogous to the above-mentioned art described in the Patent Document 2. For instance, although this usually performs the intermittent reception in a basic cycle, this varies the time intervals of intermittent reception when neither origination of a call nor an incoming call occurs for a prescribed time period, or varies the intervals on every constant or variable prescribed time period.
However, the art described in the Patent Document 1 to which the CSMA is applied has a problem, the so-called “hidden terminal problem”, where wireless signals transmitted from the respective base stations overlap with each other at the terminal and thereby the terminal becomes unable to receive the wireless signals when communication paths between base stations are not secured. If the problem arises, the system becomes equal to the simple random access system shown in the above-mentioned Non-Patent Document 2.
For instance, according to the system described in the Non-Patent Document 2, when the “hidden terminal problem” arises, transmission of call signals from two base stations during the basic frame time period increases the possibility of overlap between the two call signals on an intermittent reception time on the reception side. Occurrence of three or more call signals unavoidably causes an unreceivable overlap. It is required that the base station having transmitted the unreceivable call signal waits for an ACK (ACKNOWLEDGE) signal from the terminal until timeout.
Furthermore, the art described in the above-mentioned Patent Document 1 is a method for a system where every terminal connected to the system is temporally synchronized. Since the method determines the time on which the call signal is transmitted in the network, there is a problem where the flexibility of transmission timing of the call signal is low.
Moreover, according to the system described in the Non-Patent Document 2, it is required that a reception status be kept on every basic frame in order to monitor the call signal from the base station as with the art in the above-mentioned Patent Document 1.
Furthermore, a cluster system (a system where a plurality of terminals are connected to each other via wires) has a problem where the cluster becomes unable to receive the call signal when some terminals of the cluster become unable to receive call signals owing to a difficulty in propagation situations such that the terminals are hidden behind a rock.
As shown in a timing chart on an example of a call signal from a base station in the related art in FIG. 19, occurrence of a certain delay in transmission time from the base station 111 to terminals 101 and 102 may cause a case where neither terminal 101 nor 102 can receive the signal from the base station 111.